October 01, 2013

Neolithic boom followed by later collapse (Shennan et al. 2013)

From the paper:

It is particularly important to note that the bust following the initial farming boom is found in two historically separate agricultural expansions, the first into Central Europe c. 7,500 years ago and the second into Northwest Europe 1,500 years later. It is possible that some of these regional declines represent out-migration to neighbouring areas rather than a real decline in numbers, for example, from the Paris Basin into Britain, but, in some cases, for example, Ireland, Scotland and Wessex, it is very clear that the rising and falling trends are roughly synchronous with one another—there is little indication of one going up as the others go down.On present evidence the decline in the initially raised population levels following the introduction of agriculture does not seem to be climate-related, but of course this still leaves open a variety of possible causes that remain to be explored in the future. One possibility is disease, as the reference to the Black Death above implies, although this would have to be occurring on multiple occasions at different times in different places, given the patterns shown. It is perhaps more likely that it arose from endogenous causes; for example, rapid population growth driven by farming to unsustainable levels, soil depletion or erosion arising from early farming practices, or simply the risk arising from relying on a small number of exploitable species32. However, these suggestions remain speculative and an autocorrelation analysis of the demographic data did not find evidence of a cyclical pattern, which would be one indicator of the operation of endogenous processes (Supplementary Fig. S7). Regardless of the cause, collapsing Neolithic populations must have had a major impact on social, economic and cultural processes.

Nature Communications 4, Article number: 2486 doi:10.1038/ncomms3486

Regional population collapse followed initial agriculture booms in mid-Holocene Europe

Stephen Shennan et al.

Following its initial arrival in SE Europe 8,500 years ago agriculture spread throughout the continent, changing food production and consumption patterns and increasing population densities. Here we show that, in contrast to the steady population growth usually assumed, the introduction of agriculture into Europe was followed by a boom-and-bust pattern in the density of regional populations. We demonstrate that summed calibrated radiocarbon date distributions and simulation can be used to test the significance of these demographic booms and busts in the context of uncertainty in the radiocarbon date calibration curve and archaeological sampling. We report these results for Central and Northwest Europe between 8,000 and 4,000 cal. BP and investigate the relationship between these patterns and climate. However, we find no evidence to support a relationship. Our results thus suggest that the demographic patterns may have arisen from endogenous causes, although this remains speculative.

9 comments:

"It is perhaps more likely that it arose from endogenous causes; for example, rapid population growth driven by farming to unsustainable levels, soil depletion or erosion arising from early farming practices"

Althgough science is not a democracy that explanation gets my vote. The same probably holds for the Neolithic boom and bust in Ireland paper although their the authors see climate as the cause. Farming allowed population increase and, sooner or later, it rises too far.

I think that besides temperatures, precipitation is a huge factor in farming success, and variations can be quite local (i.e., Northern Norway is not a good proxy). In the north, farming was already quite marginal, because all the introduced species came from much warmer and dryer areas with little frost and no relevant winter snow coverage. Too much rain in the spring would have delayed sowing (not artificially drained fields partially submerged) to a degree that crops may not have had enough time to ripen; too much rain around harvest time could have destroyed much of the crop. Furthermore, almost all sites from Central Europe to the Paris basin were along water courses (which is probably also how people spread) and in low-lying valleys. We see every year how devastating floods can be, even today. Then, a 100-year flood or a 500-year flood could have wiped out an entire region (think about the floods along the Danube and Saale/Elbe this year). I believe when we see in the record that cultures or novel sub-groubs of cultures spread into a new area, it is very likely a region that was devastated by natural disasters or epidemics, rather than violent intrusion - at least until the middle bronze age/ beginning of the iron age.

Finally, for LBK it has been shown that grazing (and hey-making) land was the limiting factor before a group moved on. It does not look like there was much deforestation, initially, and people may not have known the practice of girdling, yet. And the forests on the rich, wet soils typically don't burn well (unlike pine forests on sandy soil). So, if a region got settled in a very short time period (as it often was), people also all may have moved on at a similar time.

On the isles, it looks like people quickly abandoned most serials and mostly concentrated on herding, instead. Again, too much rain can do that.

I am sure there are confounding variables the authors were not able to control for like cremation vs. burial etc. but this is a good starting point for future studies to build off of.

While this growth model may represent the growth of Europe in general, it does not represent the growth of ancestors of modern Europeans. The growth rate of the modern European gene pool would be much more dramatic as there was population replacement. ie, many of these individuals whose remains are being accounted for in this study had little to not genetic impact on modern Europeans. Therefore, using this data as a proxy for growth rate in genetic models would still yield inaccurate results.

In the paper the authors pretty much exclude climatic reasons for the rapid population decline starting around 5.300 BC. I wouldn´t be so fast with this conclusion, they didn´t find correlations with their proxis but it is correlating exactly with the Piora Oscillation.In german also known as the Rotmoos Schwankungen, the alpine treeline declined and glaciers advanced to a size only reached again at around 1850.

There's another option when it comes to explaining population reduction and that is emigration. Currently it is an issue in European economic planning.

The researchers should look south for simultaneous increases in population -- if that's possible.

It may have had nothing to do with conditions in NW and Central Europe. Things going on in the south may have attracted these people back to where their ancestors came from in the Balkans or to Italy, etc.

We tend to think of farmers as sedentary, despite good evidence that they will travel fairly long distances to market. Also there is also the almost invisible factor of the migrant farm worker.

New economic advantage in the south of Europe may even be the first thing to look for in evaluating these drops in population in the north.

If there really is a pretty universal pattern in the archaeology of a Neolithic revolution population bust, followed by a population bust and regression, in more or less independent instances of local Neolithic revolutions, then neither climate nor emigration fit the data.

The paper itself notes that it can't match a climate event to some of the bust periods. But, in cases like Ireland, emigration has to be counter-factual.

One can imagine a skipping stone pattern of Neolithic revolution booms, followed by collapse and migration, that would fit, for example, a scenario in which primate farming depletes local top soils and early Neolithic farmers lack techniques like legumes planting and fallow field rotations to engage in sustainable farming so when fields fail due to soil depletion, people move to the next region over. Copper age societies that have mastered these techniques, in contrast, can be permanent with stable urban centers and farm estates for centuries, not just a generation or two, so they escape the boom-bust cycle of first wave Neolithic people.

But, the adoption of farming leads to population booms, but also to first time exposure to animal carried diseases that jump to humans. The fact that first wave Neolithic individuals are less healthy than prior hunter-gatherers either suggests that they were more vulnerable to disease or could be evidence of Neolithic specific diseases sweeping through communities for the first time with devastating effect.

"for example, a scenario in which primate farming depletes local top soils"

This is my (current) view.

Sedentary agriculture requires techniques to replenish the soil. Population growth in that context visibly bumps up against its limit when there is physically no more land.

But what happens if you have strong population growth among a population of shifting agriculturalists?

If each village has 2-4 times as much land as they need for each year and shift location every few years to allow the land to replenish then population increases might not bump up against their limits so obviously - especially if populations like that supplement their diet with hunting and gathering.

So i wonder if under certain conditions shifting agriculture can produce sudden dramatic collapses?

Then the people in the hills (who relied more on herding animals, hunting and gathering because crops didn't grow as well in their environment) spread down into the newly deserted valleys.

@ andrew "But, in cases like Ireland, emigration has to be counter-factual."

I'm not sure about Ireland, but the economics of the facts reported in the article match those observed in the labor market in modern times.

Land opens up causing a drain on the "labor supply" in more populated areas, especially market centers. The cost of labor in those areas goes up. This increase in pay to the laborer causes a reverse movement in the direction of the more populated markets. This produces a cyclic pattern because working the land becomes more attractive as the price of labor goes down again.

You'll really cannot evaluate the presence of emigration in this case unless you know how populations varied in the areas at the other end of the shift.

That is a serious hole in this research. And the researchers admit there is a hole.

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